THE HISTORY OF MEASUREMENT -- THE VARIOUS SOURCES OF DATA, THE CHOICE OF DATA, THE QUALITATIVE APPRAISAL OF OFFENSES, AND HOW THESE MATTERS HAVE BEEN CONSIDERED BY THOSE CONCERNED WITH JUVENILE DELINQUENCY -- IS PRESENTED, FOLLOWED BY A DEFINITION OF DELINQUENCY IN LEGAL TERMS. THE ORGANIZATION OF THE JUVENILE AID DIVISION OF THE PHILADELPHIA POLICE DEPARTMENT IS THEN DESCRIBED, SINCE THE RESEARCH DATA WERE DRAWN FROM ITS RECORDS AND THE INTERPRETATION OF THESE DATA REQUIRES A KNOWLEDGE OF HOW THEY CAME TO BE RECORDED. THE RESEARCH DESIGN IS BASED ON A NUMBER OF THEORETICAL ASSUMPTIONS, WHICH ARE DISCUSSED AT LENGTH, FOLLOWED BY A DESCRIPTION OF THE DRAWING OF THE 10-PERCENT SAMPLE OF JUVENILE OFFENSES AND ITS COMPARISON WITH A SIMILAR SAMPLE BASED ON THE TOTAL OF JUVENILE OFFENSES KNOWN TO THE POLICE. OPERATIONAL DEFINITIONS OF THE CATEGORIES OF DATA TO BE TRANSCRIBED FROM THE OFFENSE REPORTS ARE OFFERED. OPERATIONAL CLASSIFICATION OF DELINQUENT CONDUCT IS THEN PRESENTED, AND ON THE BASIS OF THIS CLASSIFICATION, A SERIES OF CHAPTERS ARE DEVOTED TO AN ANALYSIS OF DISCOVERER OF THE OFFENSES, THE SEQUENCE AND LOCUS OF DISCOVERY, AND OFFENSES INVOLVING BODILY INJURY AND PROPERTY LOSS OR DAMAGE. THE BELIEF THAT MEASUREMENT OF DELINQUENCY MUST TAKE INTO ACCOUNT BOTH THE FREQUENCY AND THE DEGREE OF SERIOUSNESS OF DELINQUENT ACTS LED TO THE DEVELOPMENT OF A DESIGN FOR THE SCALING OF OFFENSES, WHICH IS FULLY DESCRIBED WITH ITS APPLICATION TO THE DATA. FINALLY, THE CONSTRUCTION OF AN INDEX OF DELINQUENCY IS DISCUSSED, ALONG WITH THE RELIABILITY AND VALIDITY OF THE SCALING RESULTS TOGETHER WITH THE THEORY OF MEASUREMENT AS RELATED TO DELINQUENCY. (AUTHOR ABSTRACT MODIFIED)